Hitherto many proposals with respect to a process for preparing a granular powder by granulating a polytetrafluoroethylene (PTFE) powder have been made. For example, JP-B-44-22619 discloses a process for stirring and granulating a PTFE powder in an aqueous medium of 30.degree. to 150.degree. C. containing a water-insoluble organic liquid having a boiling point of 30.degree. to 150.degree. C.
In order to improve the process disclosed in the above-mentioned patent publication, JP-B-5 7-15128 discloses a process in which an equipment having mechanism for pulverizing a PTFE powder is employed.
As a process for granulating by using water only, for example, JP-B-43-8611 discloses a process for granulating in which a PTFE powder is stirred in water of 40.degree. to 90.degree. C., JP-B-47-3187 discloses a process for granulating in which a PTFE powder is stirred in an aqueous medium of not less than 40.degree. C., and further JP-A-3-259926 discloses a process for granulating in which a PTFE powder is stirred in an aqueous medium in combination use of pulverizing mechanism.
Any of the processes disclosed in the above-mentioned patent publications use a so-called finely pulverized PTFE powder having an average particle size of less than 100 .mu.m. The reason why the finely pulverized powder is used is that when a coarsely pulverized powder is used, a molded article obtained therefrom is poor in physical properties, for example, tensile strength.
It is possible to pulverize a PTFE powder to an average particle size of about 100 .mu.m with a coarsely pulverizing machine, but when pulverizing to an average particle size of less than 100 .mu.m, another finely pulverizing step is separately necessary.
However, the finely pulverizing machine is expensive, and demands a larger energy consumption because machine size is large. Further there are required a pneumatic equipment for feeding the PTFE powder to the finely pulverizing machine, an auxiliary equipment for pneumatic line, a bag filter for collecting the obtained finely pulverized powder and the like.
For the mentioned reasons, it is strongly required that the granulation is conducted by using a coarsely pulverized powder having an average particle size of not less than 100 .mu.m as a PTFE powder to produce a PTFE granular powder giving a molded article having good physical properties.
As a granulation process with a coarsely pulverized powder, there is no process but a process disclosed in, for example, JP-A-3-259925, which is a process for granulating in which a PTFE powder of 420 .mu.m particle size is stirred in an aqueous medium of 60.degree. to 100.degree. C. in combination use of pulverizing mechanism.
Even by the process disclosed in the above-mentioned patent publication, the obtained PTFE granular powder does not sufficiently satisfy physical properties of a molded article such as tensile strength, and for commercial production, a further step such as a gelling and pulverizing step is required. Also since a molded article obtained by molding the granular powder has a low break down voltage, it cannot be used for applications requiring a high break down voltage. Further as mentioned above, that process requires the pulverizing mechanism.
The present inventors have studied, from various aspects, conventional processes for preparation of a PTFE granular powder by using a finely pulverized powder of tetrafluoroethylene homopolymer, and as a result, have found that use of modified PTFE, particularly perfluoro(vinyl ether)-modified PTFE enables a granulation process of a coarsely pulverized powder to give an excellent flowability, that is, handling property and a good apparent density, to a granular powder and enables a granulation process in which only water is used. The thus obtained modified PTFE granular powder gives a molded article having an excellent break down voltage.
An object of the present invention is to provide a process for preparing, without using a finely pulverized powder, a modified PTFE granular powder being excellent in handling property, i.e. flowability and in apparent density and giving a molded article having excellent physical properties.
Another object of the present invention is to provide a process in which granulation is carried out without using an organic liquid, but with water only.
Further object of the present invention is to provide a process for preparing a modified PTFE granular powder which gives a molded article having break down voltage equal to that of a molded article obtained from finely pulverized powder of unmodified PTFE, even if the modified PTFE granular powder is obtained from a coarsely pulverized powder.